We propose to study four morphological types of variation which we have identified in brains of mice of different inbred strains. Three of the types of variation are in Ammon's Horn, and they involve (1) the arrangement of pyramidal neurons in CA3 of the hippocampus, (2) the number of neurons in the granule cell layer of the Area Dentata in sections from the more dorsal, anterior portions of Ammon's Horn, and (3) the total number of neurons in all fields of Ammon's Horn in sections from the ventral portion. The fourth type of variation is in the septum, and it involves a reduction in the size of the lateral septal nucleus. Our general aims are to study how these variations are inherited, what behavioral differences may be associated with them, and what developmental events may be involved in producing these morphological variations. In order to accomplish these aims, a combination of genetic, behavioral, and morphological techniques will be required. The genetic techniques include selection and backcrossing. The behavioral techniques include measures of activity, reversal learning, and active and passive avoidance learning. Morphological techniques include studies at both the light and electron microscopic levels as well as tritiated thymidine autoradiographic studies to determine neuron "birthdasy". We hope to establish the value of genetically-associated morphological variations as precise experimental tools for the study of the functional involvements of brain structures in behavior, and for the identification and description of some major events in neural ontogeny.